Technical Field
This invention relates to the rolling of strip material. It has particular, but not exclusive, application to hot rolling of steel strip produced from a continuous caster such as a twin roll caster.
In a twin roll caster molten metal is introduced between a pair of contra-rotated horizontal casting rolls which are cooled so that metal shells solidify on the moving roll surfaces and are brought together at the nip between them to produce a solidified strip product delivered downwardly from the nip between the rolls. The term xe2x80x9cnipxe2x80x9d is used herein to refer to the general region at which the rolls are closest together. The molten metal may be poured from a ladle into a smaller vessel or series of vessels from which it flows through a metal delivery nozzle located above the nip so as to direct it into the nip between the rolls, so forming a casting pool of molten metal supported on the casting surfaces of the rolls immediately above the nip. This casting pool may be confined between side plates or dams held in sliding engagement with the ends of the rolls.
After leaving the caster the hot strip may be subjected to in-line treatment such as a controlled temperature reduction, reduction rolling, full heat treatment or a combination of such treatment steps before passing to a coiler. The coiler and any in-line treatment apparatus generally applies substantial tension to the strip which must be resisted. Moreover, it is necessary to accommodate differences between the casting speed of the twin roll caster and speed of subsequent in-line processing and coiling. Substantial differences in those speeds may develop particularly during initial start-up and until steady state casting speed is achieved. In order to meet these requirements it has been proposed to allow the hot strip leaving the caster to hang unhindered in a loop from which it passes through one or more sets of pinch rolls into a tensioned part of the line in which the strip is subjected to further processing and coiling. The pinch rolls provide resistance to the tension generated by the down-line equipment and are also intended to feed the strip into the down-line equipment.
A twin roll strip casting line of this kind is disclosed in U.S. Pat. No. 5,503,217 assigned to Davy McKee (Sheffield) Limited. In this casting line the hot metal strip hangs unhindered in a loop before passing to a first set of pinch rolls which feed the strip though a temperature control zone. After passing through the temperature control zone the strip passes through further sets of pinch rolls before proceeding to a coiler. It may optionally be hot rolled by inclusion of a rolling mill between the subsequent sets of pinch rolls.
As noted in U.S. Pat. No. 5,503,217, strip passing from zero tension to a tension part of a processing line can wander from side to side. This is not acceptable and is overcome by the first set of pinch rolls being used to steer the metal strip into the tensioned part of the processing line. However, it has been found that standard pinch rolls are not properly effective to steer the strip and hold it against the tendency to wander. The pinch rolls can in fact contribute to misalignment and lateral movement of the strip if even small variations develop in the strip to roll contact pressure, the gap between the pinch rolls, or in the profile or cross-section of the cast strip passing between them.
Wandering of the strip not only results in misalignment of the strip in the down-line processing equipment, but it can also lead to the transmission of twisting forces back into the hot strip issuing from the casting rolls. This twisting is particularly critical given the strip is at temperatures close to liquidus and thus the strip has little hot strength. In ferrous metal strip these temperatures are well in excess of 1100xc2x0 C. Thus such twisting can lead to hot lateral tearing of the strip just below the roll nip. In addition, the generation of substantial fluctuations in the tensile forces at the edge margins of the strip leads to waviness in the strip margins and the generation of small edge cracks as the strip approaches the pinch rolls. In extreme cases it can even initiate severe lateral mechanical cracking and complete disruption of the strip. Accordingly, wandering of the strip in advance of the pinch rolls is a critical problem, particularly in the casting of ferrous metal strip. Our earlier Australian Patent Application 84245/98 describes apparatus which can be applied to the steering of the strip in these circumstances to prevent excessive wandering and skewing of the strip. In that case the pinch rolls are operated to grip the strip with varying intensity across the strip to steer the strip in accordance with a control signal generated by monitoring the position of the strip in the vicinity of the pinch rolls to detect changes in lateral position of the strip and the lateral traversing velocity of skew of the strip.
The arrangement disclosed in Patent Application 84245/98 is quite satisfactory for feeding strip into most in-line treatment apparatus. However, it has been found on feeding a strip through an in-line hot rolling mill that the rolling mill itself can generate lateral movements of the strip and/or tension disturbances which can under some conditions overcome the steering control provided by the pinch rolls. The present invention provides a method and apparatus by which a strip passing between reduction rolls can be accurately steered so as to be maintained in a substantially steady straight line path. Although the invention has arisen from the need to control strip issuing from a twin roll caster into an in-line rolling mill, it could be applied in other applications where strip material is to be passed through a reduction mill.
According to the invention, there is provided a method of rolling strip material comprising passing the strip through a steering device into a rolling mill having a pair of strip reduction rolls extending laterally of the strip feed direction, the steering device being operable to steer the strip as it passes to the rolling mill, applying strip reduction squeezing forces to the reduction rolls at positions spaced apart longitudinally of those rolls, monitoring the position of the strip at a first location in the vicinity of the steering device and at a second location in the vicinity of the rolling mill, and operating the steering device and varying the relative squeezing forces on the reduction rolls at the spaced positions in response to observed positions of the strip at said first and second locations.
The steering device may comprise a pair of pinch rolls extending laterally of the strip feed direction. In that case the method may comprise passing the strip between those pinch rolls, applying strip gripping forces to the pinch rolls at positions spaced apart along the pinch rolls, and varying the relative strip gripping forces applied at said positions along the pinch rolls thereby to steer the strip. Alternatively, the pinch rolls may be moved relative to one another or they may be moved laterally of the feed direction of the strip to produce the requisite steering action.
The steering device may be operated in response to a control signal produced by comparing an observed strip position at said first location with a desired or set position. Variation of the relative squeezing forces applied to the reduction rolls may be controlled by a further control signal dependent on observed strip positions at both first and second locations when compared with desired or set positions for those locations to effect a change in the strip position and/or skew of the strip.
Said first location may be in advance of the steering device in relation to the strip feed direction and said second location may be in advance of the reduction rolls.
The invention also provides an apparatus for rolling strip material, having
a pair of reduction rolls to receive the strip material;
a roll thruster to apply squeezing forces to the reduction rolls at positions spaced apart along those rolls;
a strip steering device to steer the strip to the reduction rolls and operable to vary the feed direction;
a first strip position sensor to monitor the position of the strip at a first location in the vicinity of the strip steering device;
a second strip position sensor to monitor the position of the strip at a second location in the vicinity of the reduction rolls; and
a controller responsive to outputs of the first and second strip position sensors to operate the steering device to vary the direction of feed to the reduction rolls and also to vary the relative squeezing forces applied to the reduction rolls at the spaced apart positions.
The relative squeezing forces applied to the reduction rolls at the spaced apart positions may vary the position and skew of the strip entering the reduction rolls.
The first and second sensors may each consist of a plurality of individual sensors.